Of the many uses for a continuously advancing mandrel surface, perhaps the most important is in the manufacture of indefinite length helically fabricated tubing. Such a mandrel is wrapped helically with strip or other fabricating elements and upon suitable attachment of the applied convolutions there emerges from the end of the mandrel a progressively advancing finished tube of indefinite length. One early attempt in the design of such a mandrel involved the use of longitudinal closed-loop belts traveling along the surface of a mandrel structure and returning through its interior. U.S. Pat. No. 2,828,239 illustrates such apparatus.
A substantial improvement over longitudinal belt mandrels is described in U.S. Pat. No. 3,155,559 and that patent still represents the most highly developed state of the art upon which the improvement of the present invention has been made. It discloses a continuously-advancing definite-length mandrel comprising a frame and a substantially tubular mandrel core axially rotatable with respect to the frame. A belt is provided with a portion thereof wrapped helically about the core with its convolutions defining a mandrel surface. Drive means are included for rotating the core with respect to the frame and drawing additional convolutions of the belt about one end of the core. Feeding means are included on the frame for urging all of the convolutions toward the opposite end of the core. A belt guide is provided at the opposite end of the core and affixed to the frame for controlling the collapse of each successive belt convolution and directing it inwardly through the core so that it can be wrapped back onto the core.
It was recognized from the outset that the helically fabricated tubing made on such a mandrel would have a helical lead or pitch equal to the lead or pitch of the single belt. Therefore it was suggested in the prior art patent itself that two or more belts could be wrapped around the core and passed around a multiple groove device so that the mandrel surface would advance further at each revolution of the core (see lines 1 to 9 of column 5 of U.S. Pat. No. 3,155,559). It is the purpose of the present invention to carry that suggestion to its optimum conclusion and thus provide a multiple belt mandrel device which is as free as possible of unequal drag and friction on its plurality of belts and which permits the widest possible belts to be used for a given mandrel diameter. The wider the belts in relation to mandrel diameter, the greater will be the lead or pitch of the helically fabricated tubing in relation to its inside diameter, which in turn is an advantage in many designs of tubing.